Rotor assembly

ABSTRACT

A rotor assembly that includes a shaft to which are mounted an impeller, a rotor core and a bearing cartridge. The bearing cartridge is mounted between the impeller and the rotor core and comprises a pair of bearings, a spring that applies a preload to each of the bearings, and a sleeve that surrounds the bearings.

REFERENCE TO RELATED APPLICATIONS

This application claims the priority of United Kingdom Application No. 0903053.7, filed Feb. 24, 2009, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a rotor assembly for a high-speed compressor.

BACKGROUND OF THE INVENTION

The rotor assembly of many high-speed compressors includes a shaft mounted to a housing by a pair of bearings, each bearing located at opposite ends of the shaft. The provision of bearings at opposite ends of the shaft affords good stability. However, the pathway between the two bearings through the housing is relatively long and typically traverses several distinct components of the compressor. As a consequence of dimensional and geometric tolerances, it is generally difficult to ensure that the rotor assembly is accurately aligned within the housing. Additionally, for high-speed compressors, the bearings are ideally preloaded to prevent skidding. However, owing to the tolerance stack associated with the pathway between the bearings, it is difficult to ensure that both bearings are preloaded with a force that prevents skidding without being excessive, which would otherwise result in poor bearing performance.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides a rotor assembly comprising a shaft to which are mounted an impeller, a rotor core and a bearing cartridge, the bearing cartridge being mounted between the impeller and the rotor core and comprising a pair of bearings, a spring applying a preload to each of the bearings, and a sleeve surrounding the bearings.

The rotor assembly is therefore supported by two preloaded bearings. The bearings are ideally spaced by a predetermined amount and the spring has a predetermined spring constant. Consequently, the two bearings are preloaded with the same, well-defined force. By preloading the bearings with a force that prevents skidding without being excessive, the lifetime of the bearings is increased.

The bearing cartridge provides a single fastening over which the rotor assembly may be secured to a housing, frame or the like. In providing a single fastening, securement of the rotor assembly is both easier and cheaper than that for a rotor assembly having bearings located at opposite ends of the shaft.

The sleeve acts to align the two bearings and in particular the outer races of the bearings. Consequently, when the rotor assembly is secured to a housing, alignment of the shaft within the housing is assured. The sleeve also acts as a heat sink for the bearings, thus prolonging the life of the bearings.

In mounting the impeller and rotor core on opposite sides of the bearing cartridge, a compact design of rotor assembly is achieved. Moreover, the rotor assembly may be dynamically balanced as a complete unit prior to inclusion within a compressor. This in contrast to other rotor assemblies that must be assembled within the compressor and cannot therefore be dynamically balanced as a complete unit. Furthermore, in having the impeller and rotor core located on opposite sides of the bearing cartridge, radial loading of the two bearings may be more evenly balanced. Indeed, to this end, the centre of mass of the rotor assembly is preferably located at a point within the bearing cartridge.

The sleeve preferably has a coefficient of thermal expansion substantially matching that of the shaft. Consequently, thermal expansion of the shaft and sleeve does not adversely affect the loading of the bearings. Additionally, the sleeve may be formed of an electrical insulating material. This then has the advantage that the sleeve is not susceptible to inductive heating that might otherwise arise during use of the rotor assembly. The rotor assembly is thus ideally suited to high-speed applications since the sleeve continues to serve as an effective heat sink for the bearings. Furthermore, the sleeve may be formed of a non-magnetic material. As a consequence, the sleeve does not present a partial short to the magnetic circuit of the rotor core and stator.

Advantageously, the rotor assembly comprises a spacer that surrounds the shaft, separates the two bearings and contributes to the shaft stiffness. Consequently, for a rotor assembly having a particular critical speed, a thinner, less massive shaft may be used. Moreover, the spacer ensures that the bearings are spaced by a predetermined amount, thereby ensuring a well-defined bearing preload. In order to minimise any possible failure of the bearing cartridge due to thermal expansion, the bearings and the spacer preferably have a coefficient of thermal expansion substantially matching that of the shaft and sleeve.

The shaft is advantageously formed of a soft magnetic material. Accordingly, the shaft reduces the reluctance of the magnetic circuit.

The bearings may be preloaded by one or more springs located between the spacer and the sleeve. However, a single coil spring is preferably employed that surrounds the spacer. This then makes assembly of the rotor assembly cheaper and easier.

When the rotor assembly is in use, rotation of the impeller may cause a reduction in pressure at the end of the bearing cartridge adjacent the impeller. This reduction in pressure may cause dirt-laden fluid to be drawn through the bearing cartridge via annular gaps in the bearings. The rotor assembly therefore preferably comprises a dirt cap that covers the annular gap in the bearing adjacent the rotor core. For example, the dirt cap may be secured to the shaft adjacent the bearing and include a flange that extends radially outward from the shaft to cover the annular gap. Consequently, as fluid is drawn towards the bearing cartridge, any dirt carried by the fluid is obstructed from entering the bearing.

In a second aspect, the present invention provides a rotor assembly comprising a bearing cartridge mounted to a shaft, the bearing cartridge comprising a pair of bearings, a spring applying a preload to each of the bearings, and a sleeve surrounding the bearings, wherein the sleeve has a coefficient of thermal expansion substantially matching that of the shaft.

The bearing cartridge provides a single fastening over which the rotor assembly may be secured to a housing, frame or the like. The provision of two spaced bearings affords relatively good stability, while the sleeve acts to align the bearings and serves as a heat sink. Since the sleeve and shaft are thermally matched, thermal expansion of the shaft and sleeve does not adversely affect the loading of the bearings during subsequent use.

In a third aspect, the present invention provides a rotor assembly comprising a bearing cartridge mounted to a shaft, the bearing cartridge comprising a pair of bearings, a spring applying a preload to each of the bearings, and a sleeve surrounding the bearings, wherein the sleeve is formed of an electrically insulating material.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood, an embodiment of the invention will now be described, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a sectional view of a rotor assembly in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The rotor assembly 1 of FIG. 1 comprises a shaft 2 to which are mounted an impeller 3, a rotor core 4, a bearing cartridge 5, and a dirt cap 6.

The impeller 3 is mounted to a first end of the shaft 2. The impeller 3 illustrated in FIG. 1 is a centrifugal impeller. However, other types of impeller may equally be employed according to the intended application of the rotor assembly 1.

The rotor core 4 is mounted to a second end of the shaft 2. The rotor core 4 is formed of a hard or soft magnetic material. The shaft 2 is formed of a soft magnetic material which then reduces the reluctance of the magnetic circuit. Nevertheless, a non-magnetic material might alternatively be used for the shaft 2.

The bearing cartridge 5 is located between the impeller 3 and the rotor core 4. The bearing cartridge 5 comprises a pair of bearings 7,8, a spacer 9, a spring 10, and a sleeve 11.

The bearings 7,8 are identical, with each comprising an inner race 12, a cage 13 supporting a plurality of ball bearings 14, an outer race 15, and a pair of shields 16 secured to the outer race 15 at opposite ends of the bearing 7,8.

The spacer 9 is cylindrical and serves to space the two bearings 7,8 by a predetermined amount. In addition to spacing the two bearings 7,8, the spacer 9 contributes to the stiffness of the shaft 2. Consequently, the material and thickness of the spacer 9 may be selected to achieve a desired shaft stiffness. In spite of the aforementioned advantages, the spacer 9 may be omitted from the bearing cartridge 5. A predetermined spacing between the two bearings 7,8 may then be achieved by alternative means during assembly of the bearing cartridge 5.

The spring 10 is a coil spring that surrounds the spacer 9 and applies a force to the outer races 15 of the bearings 7,8. Since the spacer 9 has a predetermined length and the spring 10 has a predetermined spring constant, each of the bearings 7,8 is preloaded with the same well-defined force. In the particular embodiment illustrated in FIG. 1, the spring 10 is significantly thicker than the outer races 15 of the bearings 7,8. In order that the force of the spring 10 is transferred to the outer races 15 in the axial direction, a circular shim 17 is provided between the spring 10 and each of the outer races 15.

The sleeve 11 surrounds and is secured to the two bearings 7,8. The sleeve 11 serves three purposes. First, the sleeve 11 provides an outer surface over which the bearing cartridge 5 may be secured (e.g. by press-fitting or gluing) to a housing, frame or the like. Second, the sleeve 11 acts as a heat sink for the bearings 7,8. Since heat generated by the bearings 7,8 is carried away by the sleeve 11, the lifetime of the bearings 7,8 is prolonged. Third, as will now be described, the sleeve 11 acts to align the bearings 7,8.

In mounting the bearings 7,8 to the shaft 2, the inner races 12 of the bearings 7,8 are coaxially aligned. Although the inner races 12 are aligned, the outer races 15 are not. This is because the outer race 15 of each bearing 7,8 has a small degree of play relative to the inner race 12. When the outer races 15 are preloaded, the force applied by the spring 10 is not normally uniformly distributed around each outer race 15. As a result, the outer races 15 are not normally aligned. In particular, the longitudinal axis of one outer race may be radially displaced and/or tilted with respect to the longitudinal axis of the other outer race. If the sleeve 11 were omitted from the bearing cartridge 5, and the rotor assembly 1 were mounted to a housing via the bearings 7,8, any misalignment in the outer races 15 would result in misalignment of the shaft 2 within the housing. This in turn would have serious consequences for the alignment of the rotor core 4 relative to a stator, as well as the alignment of the impeller 3 relative to a shroud and diffuser held within the housing. In securing a sleeve 11 over the outer races 15 of the two bearings 7,8, the outer races 15 are brought into coaxial alignment. Consequently, when the bearing cartridge 5 is mounted to the housing, correct alignment of the shaft 2 within the housing is assured.

When the rotor assembly 1 is in use, the sleeve 11 of the bearing cartridge 5 is exposed to a varying magnetic field from the rotor core 4 and/or the stator used to drive the rotor core 4. In order that the sleeve 11 is not subjected to inductive heating and does not present a partial short-circuit to the magnetic circuit of the rotor core 4 and stator, the sleeve 11 is formed of a non-magnetic, electrically insulating material. Nevertheless, there may be applications for which a magnetic and/or electrically conducting material may be used, e.g. if the magnetic field strength of the rotor core 4 and/or stator is relatively weak, if the rotor assembly 1 rotates at relatively low speed, if the bearing cartridge 5 is spaced sufficiently far from the rotor core 4, or if the mass of the sleeve 11 exposed to the magnetic field is relatively small.

The components of the bearing cartridge 5 (i.e. the bearings 7,8, spacer 9, spring 10 and sleeve 11) have coefficients of thermal expansion that substantially match that of the shaft 2. This then prevents failure of the bearing cartridge 5 due to uneven thermal expansion. Nevertheless, thermal matching of one or more components may not be necessary, particularly if the temperature range over which the rotor assembly 1 operates is relatively small. Since the bearings 7,8 are secured to the shaft 2 and sleeve 11, uneven thermal expansion of the shaft 2 and sleeve 11 will cause the inner race 12 of each bearing 7,8 to move relative to the outer race 15. This in turn may lead to adverse changes in the preload of the bearings 7,8. By ensuring that at least the shaft 2 and sleeve 11 are thermally matched, adverse changes in the preload may be averted.

The dirt cap 6 is secured to the shaft 2 adjacent the end of the bearing cartridge 5 proximate the rotor core 4. The dirt cap 6 comprises a collar 18 having a flange 19 formed at one end. The collar 18 is secured to the shaft 2 such that the flange 19 abuts the inner race 12 of the bearing 7. The flange 19 extends radially outward from the shaft 2 to cover the annular gap in the bearing 7 between the inner race 12 and the shield 16, the advantage of which will now be described.

When the rotor assembly 1 is in use, rotation of the impeller 3 creates a reduction in pressure beneath the impeller 3. The pressure at the rotor core 4, however, is unchanged. Consequently, a pressure gradient is created along the length of the bearing cartridge 5 that encourages fluid to flow through the bearing cartridge 5. Fluid enters the bearing cartridge 5 at the gap between the inner race 12 and the shield 16 of the bearing 7 adjacent the rotor core 4. Any dirt carried by the fluid is likely to be trapped by the bearing 7 and thus impair performance and ultimately lead to premature failure of the bearing 7. The dirt cap 6 reduces the amount of dirt entering the bearing 7 by forcing the fluid to follow a convoluted pathway around the flange 19. This convoluted pathway causes most dirt to be thrown radially outward away from the bearing 7. In addition, the space between the flange 19 and the bearing 7 is sufficiently small that, as the flange 19 rotates relative to the shield 16, a reduced pressure region is created between the flange 19 and bearing 7. Consequently, the pressure gradient across the bearing cartridge 5, and thus the flow of fluid through the bearing cartridge 5, is reduced. In reducing the amount of dirt entering the bearing 7, the lifetime of the bearing is increased, particularly when the rotor assembly 1 is employed in dirt-laden environments.

Although each bearing 7,8 is provided with a pair of shields 16, the interior of the bearing cartridge 5 is not directly exposed to the external environment. Accordingly, the shield 16 at the end of each bearing 7,8 adjacent the spacer 9 and spring 10 may be omitted. Nevertheless, each shield 16 provides resistance to the flow of fluid through the bearing cartridge 5 and thus the provision of shields 16 at either end of each bearing 7,8 aids in minimising the amount of dirt entering the bearing cartridge 5.

In the embodiment described above, the shields 16 are secured to the outer race 15 of each bearing 7,8. The dirt cap 6 therefore covers an annular gap formed between the inner race 12 and the shield 16 of the bearing 7 adjacent the rotor core 4. Alternative designs of bearing 7 are, however, possible in which the shields 16 are sealed to the inner race 12 or omitted altogether. As a result, an annular gap may alternatively exist between the shield 16 and the outer race 15 or between the inner race 12 and the outer race 15. Should an alternative design of bearing be employed, the diameter of the flange 19 may be adapted such that the dirt cap 6 continues to cover any gap.

The rotor assembly 1 of the present invention offers several advantages over known rotor assemblies, particularly those in which a shaft is supported by end bearings. With the rotor assembly 1 of the present invention, the shaft 2 is supported by two bearings 7,8 that are spaced apart to provide good stability. Any reduction in stability as a result of supporting the shaft 2 at its centre rather than at its ends may be compensated by the spacer 9, which contributes to the shaft stiffness. The bearings 7,8 of the rotor assembly 1 are preloaded with the same, well-defined force, thus prolonging the life of the bearings. This is in contrast to rotor assemblies having a shaft supported by end bearings, in which the tolerance stack associated with the bearings makes accurate preloading difficult. A further advantage of the rotor assembly 1 lies in the provision of the sleeve 11, which serves to align the bearings 7,8. Consequently, when the rotor assembly 1 is secured to a housing, alignment of the shaft 2 within the housing is assured. Again, this is in contrast to rotor assemblies having a shaft supported by end bearings, in which the tolerance stack makes accurate alignment of the shaft within the housing difficult. In addition to aligning the bearings 7,8, the sleeve 11 acts as heat sink for the bearings 7,8, thus further prolonging their life. The bearing cartridge 5 provides a single fastening over which the rotor assembly 1 may be secured to a housing. Consequently, securement of the rotor assembly 1 is both easier and cheaper than that for rotor assemblies having end bearings. In mounting an impeller and a rotor core to a shaft on opposite sides of a bearing cartridge, the present invention provides a compact design of rotor assembly. Moreover, since two masses are located on opposite sides of the bearing cartridge, radial loading of the bearings may be more evenly balanced, thus prolonging bearing life. This in contrast to other rotor assemblies in which the bearings typically undergo different radial loading. Finally, the rotor assembly 1 of the present invention may be dynamically balanced as a complete unit (i.e. shaft, impeller, rotor core and bearing cartridge) prior to inclusion within a compressor. This in contrast to other rotor assemblies that must be assembled within the compressor and cannot therefore be dynamically balanced as a complete unit. 

1. A rotor assembly comprising a shaft to which are mounted an impeller, a rotor core and a bearing cartridge, the bearing cartridge being mounted between the impeller and the rotor core and comprising a pair of bearings, a spring applying a preload to each of the bearings and a sleeve surrounding the bearings.
 2. A rotor assembly as claimed in claim 1, wherein the sleeve has a coefficient of thermal expansion substantially matching that of the shaft.
 3. A rotor assembly as claimed in claim 1, wherein the sleeve is formed of an electrically insulating material.
 4. A rotor assembly as claimed in claim 1, wherein the sleeve is formed of a non-magnetic material.
 5. A rotor assembly as claimed in claim 1, wherein the centre of mass of the rotor assembly is located within the bearing cartridge.
 6. A rotor assembly as claimed in claim 1, wherein the rotor assembly comprises a spacer that surrounds the shaft, separates the two bearings and contributes to the shaft stiffness.
 7. A rotor assembly as claimed in claim 6, wherein the bearings and the spacer have a coefficient of thermal expansion substantially matching that of the shaft.
 8. A rotor assembly as claimed in claim 1, wherein the shaft is formed of a soft magnetic material.
 9. A rotor assembly as claimed in claim 1, wherein the rotor assembly comprises a dirt cap that covers an annular gap in the bearing adjacent the rotor core, the dirt cap being secured to the shaft and comprising a flange that extends radially outward from the shaft to cover the annular gap.
 10. A rotor assembly comprising a bearing cartridge mounted to a shaft, the bearing cartridge comprising a pair of bearings, a spring applying a preload to each of the bearings, and a sleeve surrounding the bearings, wherein the sleeve has a coefficient of thermal expansion substantially matching that of the shaft.
 11. A rotor assembly as claimed in claim 10, wherein the sleeve is formed of an electrically insulating material.
 12. A rotor assembly as claimed in claim 10, wherein the sleeve is formed of a non-magnetic material.
 13. A rotor assembly as claimed in claim 10, wherein the shaft is formed of a soft magnetic material.
 14. A rotor assembly as claimed in claim 10, wherein the rotor assembly comprises a spacer that surrounds the shaft, separates the two bearings and contributes to the shaft stiffness.
 15. A rotor assembly as claimed in claim 14, wherein the bearings and the spacer have a coefficient of thermal expansion substantially matching that of the shaft.
 16. A rotor assembly as claimed in claim 10, wherein the rotor assembly comprises a dirt cap that covers an annular gap in one of the bearings, the dirt cap being secured to the shaft and comprising a flange that extends radially outward from the shaft to cover the annular gap.
 17. A rotor assembly comprising a bearing cartridge mounted to a shaft, the bearing cartridge comprising a pair of bearings, a spring applying a preload to each of the bearings, and a sleeve surrounding the bearings, wherein the sleeve is formed of an electrically insulating material.
 18. A rotor assembly as claimed in claim 17, wherein the sleeve is formed of a non-magnetic material. 